Inspectors Dig Into Underground Duct Codes

MIDLAND, MI — A packed room of members of the Mechanical Inspectors Association of Michigan (MIAM) recently gathered to listen to and discuss the many aspects of the proper installation of underground duct systems. This was just one of several seminars presented at the MIAM Fall 2002 Educational Conference in Midland.

Scott Paddock, mechanical inspector of Oshtemo Township, MI, and a licensed HVACR contractor, explained some of the highlights of the Michigan Residential Code (MRC) M1601.1.2: “Underground Duct Systems.”

“Understanding codes is a lot easier if you read them one line at a time,” Paddock said. He said that he often has to talk with contractors about the proper way to fill in and cover underground duct.

“Some contractors don’t care what is in that shovel full of dirt they are throwing over the duct,” he said.

Paddock reminded inspectors that protecting underground duct during the construction process was as important as the workmanship of the duct itself. “Make sure builders are taking precautions to protect underground duct from heavy equipment going over it,” he noted. “For example, ground ramps should not be built out of wood.”

Paddock listed some standards endorsed by the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA):

  • Porous fill and earth fill should not be dumped directly on ducts in trenches.

  • Fill should be firmly but not heavily tamped under and around the duct.

  • The first foot of fill should be shoveled on top of the duct.

  • Fill should not contain stones larger than two inches.


    The MRC reads: “Underground duct systems shall be constructed of approved concrete, clay, metal, or plastic. For plastic ducts, the maximum duct temperature shall not exceed 150 degrees. Metal ducts shall be protected from corrosion in an approved manner or shall be completely encased in concrete not less than two inches thick.”

    Paddock said, “PVC ductwork is already corrosive resistant and does not have to be encased in concrete. Plastic duct should be run underground, down line, and metallic duct should be run closer to the source where there is high heat.”


    One discussion that drew a lot of interest at the meeting was the location of floor vents. Section 603.15.2 of the MRC addresses floor flooding. It stipulates that floor registers or baseboard registers may not be located in toilet rooms, bathrooms, washrooms, laundry rooms, utility rooms containing water outlets, kitchens, basements, and pool or spa rooms.

    The code notes an exception: “Baseboard registers may be installed if the duct penetrates the floor 1¼2 of an inch and if the register is sealed at the floor.”

    “No one can tell me why you can’t have a floor register in a basement,” Paddock said. “And it may come up for a code change.”

    Phil Forner, a Michigan HVACR contractor and mechanical inspector, said code interpretations such as defining a basement are sometimes a little too “fuzzy,” which is one reason why groups like MIAM meet to discuss the standards.

    “Michigan is moving toward one set of standards with more definitive wording,” Forner said. “After all, the more wording means the standard is open to less interpretation.”

    Publication date: 10/21/2002

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